1. Field of the Invention
The present invention is directed to methods for making emulsions having a high ratio of internal (dispersed) phase to external (continuous) phase. More specifically, in accordance with the methods of the present invention, silicone in water emulsions are continuously made with a low probability of phase inversion.
2. Description of Related Art
An emulsion is a dispersion of one liquid phase in another substantially immiscible continuous liquid phase. As these dispersions have a tendency to separate, stabilizing agents, i.e., emulsifiers, are added to maintain the emulsion. Emulsions having a high ratio of dispersed phase to continuous phase are known in the art as High Internal Phase Ratio Emulsions (“HIPRE” “HIPR emulsions” or “HIPREs”) or High Internal Phase Emulsions (“HIPE” or “HIPEs”). These emulsions can be either oil-in-water or water-in-oil emulsions and typically contain more than 70% internal, or dispersed, phase.
HIPR oil-in-water emulsions are normally prepared by dispersing oil into the continuous phase under conditions capable of producing emulsions wherein the internal phase volume exceeds 70%. Clearly, for very high internal phase volumes the systems cannot contain discrete spherical oil droplets; rather, they consist of highly distorted oil droplets separated by thin interfacial aqueous films.
Several devices are known for the preparation of HIPR emulsions. Of these devices, some involve a batch preparation procedure, such as those disclosed in U.S. Pat. No. 4,934,398. In addition, the art has typically used mixers that use rotating elements to provide the shear necessary to disperse the internal phase throughout the continuous phase in producing HIPR emulsions. See, e.g., U.S. Pat. Nos. 5,250,576 and 5,827,909. Other methods include those of U.S. Pat. No. 3,684,251, which discloses a series of stacked consecutive-flow mixing chambers of progressively increasing dimensions possessing agitating means for the continuous production of emulsions.
Rotor-stators are frequently used to prepare HIPR emulsions. Although rotor-stators can be operated continuously they are generally designed for use in a single-pass mode. Therefore, the dispersed and continuous phases are fed into the rotor-stator in the ratio required for the final emulsion, e.g., 80% oil and 20% continuous phase. The only way the rotor-stator can disperse this large portion of oil into the much smaller portion of continuous phase is to use very high shear. As the emulsion forms in the rotor-stator unit the high shear rate generates a large amount of heat, which is undesirable for many products, and the high shear rate required to form the emulsion is much higher than what is required to reduce the particle size of the oil droplets in the emulsion. Thus, much of the heat generated in the rotor-stator is unnecessary. In addition, for many emulsions the rotor-stator causes phase inversion by dispersing the continuous phase into the oil phase, which does not result in a final product with desired characteristics. All particle sizes in the present invention refer to average or mean particle size and more specifically to volume-average particle size.
Other means of applying shear that can be more precisely controlled are in-line mixers or static mixers. In such mixers, the fluid flow past fixed elements is divided and recombined by the arrangement of the elements to provide mixing.
Methods have been developed for the continuous production of HIPR emulsions. For example, U.S. Pat. Nos. 4,472,215 and 4,844,620 disclose processes for the continuous production of HIPR emulsions wherein the phase materials are introduced into a recirculation line in the system.
Methods for making HIPR emulsions consisting of silicone in water are also known in the art. For example, U.S. Pat. Nos. 5,563,189 and 5,665,796 describe a continuous process for making high-internal-phase-ratio emulsions (HIPRE's) consisting of silicone oil in water utilizing high-shear rotor-stators to disperse a high volume of oil in a low volume of water.
It would be desirable to provide a continuous system for forming an HIPR silicone oil-in-water emulsion without prohibitive amounts of mixing energy or emulsifiers, and without causing high temperatures or phase inversion.